Mechanical air control equipment of a Heating Ventilation and Air Conditioning (HVAC) system can be a major source of sound in a building. The sound generated by the HVAC system travels both upstream and downstream in the intake air duct and exhaust air duct, respectively. Various sound sources within the duct include blower fans, diffusers, airflow regulating valves, etc. Noise generated by the blower fans is of broad band frequency, typically ranging from about 200 Hz through the rest of the frequency range of human hearing. Therefore, noise generated by HVAC system, when travelling upstream through the very short intake air duct, exits the intake air filter and enters the building living area creating a noise pollution problem.
A cost-efficient HVAC system often requires placing the HVAC heat exchange system in the middle of a building. This exchange system is often placed in a hallway, which frequently leads to a family room, dining room, bedroom, kitchen, etc. Noise generated by the HVAC blower fan and other mechanical parts travels through the short section of the intake air duct and enter the living space. This noise pollution makes normal conversation and comfortable television listening difficult.
Various attempts have been made to minimize the sound generated by an HVAC system. However, these attempts generally address the treatment at the exhaust air duct section, and not the treatment at the intake air duct section. Because the intake air duct section is usually a very short section of the air duct, it leaves very little space for any effective noise treatment. One exhaust air duct treatment system is commonly referred to as a dissipative silencer, which provides a noise attenuating liner either inside or outside the duct. This liner may be mineral wool or fiberglass insulation. These materials moderately attenuate sound over a broad range of frequencies. However, these liners are often not desirable because of large space requirements and the extended length of coverage that is required to produce adequate attenuation.
Additionally, reactive silencers have been used to attenuate sound. They typically consist of perforated metal facings that cover a plurality of tuned chambers. Generally, reactive silencers attenuate low frequency noise. Broad band attenuation is more difficult to achieve with reactive silencers, due to the larger area required to achieve a noticeable result. Another way to reduce the noise in an exhaust duct is by employing an acoustic resonator. This technique includes at least one resonating chamber having walls that define a length and a height. The length of the resonating chamber is selected to provide noise attenuation at a predetermined frequency. Generally, acoustic resonators only attenuate a predetermined frequency or frequencies. To achieve broad band frequency attenuation with an acoustic resonator requires a large number of different lengths and sizes of resonating chambers, which, in turn, requires a large area and volume of work space.
All of the above mentioned noise-attenuating techniques require a large area and volume of workspace which is only possible in the exhaust duct area. The usually short intake air duct provides too small an area for the previous designs to effectively attenuate broad band HVAC noise.
Another attempt to reduce noise is by active noise attenuation. This is accomplished by sound wave interference. Undesirable noise propagating within a duct is attenuated by the introduction of a canceling sound. An input microphone typically measures the undesirable noise up stream in a duct and converts it to an electrical signal. The signal is processed by a digital computer that generates a sound wave of equal amplitude and 180 degrees out of phase (a mirror image of the noise). This secondary noise source destructively interferes with the noise and cancels a significant portion of the unwanted noise. However, the adaptive process that is used to generate the canceling signal can be adversely affected by acoustical reflection from distant elements in the overall duct system. Active attenuation is only useful on low frequency (below about 100 Hz) noise attenuation and is not efficient in attenuating higher frequencies. Additionally, the high cost of this system further limits its use.